heterodyne: Correct threshold recipe parameters.

 - Although connected BASELINE_LINEARITY_MINRMS should not require
   BASELINE_LINEARITY_CLIP to be set to be accessed, since the
   primitive _MASK_BAD_BASELINE_LINEARITY_ has a valid default.
 - Set the default of linearity clip to be undef as documented.
   Always being defined meant that BASELINE_LINEARITY_MINRMS and
   BASELINE_LINEARITY_CLIP were effectively ignored.
heterodyne: Correct typing errors in documentation and comments.